The present invention relates to urea fertilizers, and more particularly to a controlled release formulation for urea fertilizers.
Fertilizers are generally classified into two major groupings, namely, natural organic products and synthetic chemical products. Synthetic chemical product examples would be urea, ammonia phosphates, and many others. Natural organic products are typically by-products from processing of animal or vegetable substances that contain plant nutrients of value as fertilizers. Currently, the three principal types of commercialized natural organic materials are activated sewage sludge, co-polymerized leather tankage, and hydrolyzed leather meal. Sludge is typically filtered off from an aeration reactor and heat dried for commercial use as a slowly soluble nitrogen source available through microbial degradation. However, because of its low nutrient value sludge is generally used only as a base for fertilizers sold particularly to the home and garden market. Various materials containing unavailable nitrogen as keratin are used to make process tankage. The principal commercial products are manufactured from leather scrap by treatment with steam under pressure. This treatment hydrolyzes the keratin to purines and amino acids which are more available nitrogen sources. A co-polymer of leather tankage and methylene urea is also available.
Controlled release fertilizers include natural organic and synthetic chemical products in which the release or availability of plant nutrients is in some way deliberately regulated so as to distribute the nutrient uptake over time. Controlled nutrient uptake can be achieved either through modification of the fertilizer product itself, e.g. reduced solubility, coating, or encapsulation, or through regulation of nutrient availability by the plant, e.g. nitrification inhibitors. In general, controlled release fertilizers have the following advantages over natural organic sources. First, controlled release fertilizers provide a reduction in the nutrient losses that occur between application and uptake by the plant. Secondly, controlled release fertilizers provide for the reduction of nitrogen toxicity, particularly to seedlings, caused by high ionic concentrations associated with rapid dissolution of soluble organic fertilizers or from the evolution of ammonia by hydrolysis of urea salts. Finally, controlled release fertilizers provide a reduction in the number of fertilizer applications necessary thus resulting in substantial cost savings.
The above advantages are particularly advantageous with respect to nitrogen sources for plants because more avenues of nitrogen loss exist than for phosphorous and potassium. For example, denitrification of anhydrous ammonia can cause volatilization losses of ammonia, and nitrogen may be removed from the root zones of plants because of leeching or other movement of these nutrients in the soil. This loss is a particular problem in porous or sandy soils, soil subject to heavy rainfall, soils with substantial ground water movement and runoff, and areas that are heavily irrigated. Also, nitrogen may become unavailable to the plant if it forms an insoluable compound in the soil.
The most common method of modifying the fertilizer product itself to provide controlled nutrient release is to control the solubility of the fertilizer. In the case of urea, such products are typically made by reacting the urea with various aldehydes to reduce the solubility of the material. For example, isobutylidene diurea (IBDU), as described in U.S. Pat. No. 3,322,528, is a condensation product of urea and isobutyraldehyde which contains about 31% nitrogen of which 90% is water insoluble. The rate of nitrogen release is a function of soil moisture and the size of the IBDU particle so that the more moisture available and the finer the particle size, the more rapid the rate of nitrogen release.
Urea has also been reacted with formaldehyde and consists mainly of methylene urea polymers varying in chain length and degree of cross linking. Nitrogen is released from the insoluble portion of these materials by microbial degradation and therefore factors such as soil moisture, temperature, pH, nutrient content and oxygen which influence the rate of microbial activity also effect the rate of nitrogen release. As with the production of IBDU, the urea actually takes part in the reaction to form urea formaldehydes.
Modification of the fertilizer product to control the amount of nutrient uptake can also be achieved by coating soluble fertilizers to meter the nitrogen release. Coatings are generally classified into one of three types. First, there are semipermeable membranes which are broken down by internal osmotic water pressure built-up by vapor diffusion. Release of the nitrogen from the soluble fertilizer is usually complete once the coating is broken. Another type of coating involves the use of impermeable membranes with small pores. In this type of coating water passes through the coating and dissolves the fertilizer, causing swelling of the capsule and enlargement of the pores. The dissolved fertilizer then diffuses through the enlarged pores in the coating. Finally, impermeable membranes without pores are utilized to coat soluable fertilizers. In this type of coating, chemical, physical or microbial action degrades the membrane material before fertilizer release occurs, and nutrient release is usually complete once the coating is degraded.
Several controlled release products are polymer coatings based on impermeable membranes with small pores to coat prilled, soluble fertilizers. Release of the nutrients can be varied by changing the thickness of the coating. The rate of release is also governed by soil temperature i.e. higher temperatures increase the nutrient release rate. One such material is a fertilizer coated with a copolymer of dicyclopentadiene with glycerol ester. Nutrient release varies with the thickness of the coating which ranges between 4% and 15% of the finished product weight. Another type of polymer coated fertilizer consists of coated urea, ammoniated superphosphoric acid, and potassium chloride. In this case, the nutrients are released through microscopic pores in the capsule wall which includes a low molecular weight polyethylene.
Sulfur coated urea has also been utilized to provide a controlled release fertilizer. Nitrogen release is based upon the thickness and completeness of the sulfur coating, the soil moisture, and the soil temperature. Increased soil moisture and temperature accelerate the degradation of the impermeable sulfur coating and thus the diffusion of urea through the pores in the coating. For such products, the rate of nitrogen release is expressed as a seven day dissolution rate. The seven day dissolution rate is measured as the percentage of urea that dissolves when a 50 gram sample of the product is immersed in 250 ml of water at 37.8.degree. C. for seven days. Typically, these products have seven day dissolution rates of between 25% and 35% which indicates a rapid initial rate of nitrogen release.
As previously mentioned, controlled release of the fertilizer nutrients may also be accomplished through nitrification inhibitors. Nitrification is the process which converts ammonium ions, when applied to the soil as ammonium nitrogen, by bacterial oxidation to nitrate ions. Certain materials inhibit nitrification because they are toxic to the soil bacteria that oxidize ammonium ions. For example certain pesticides and chemicals such as nitropyrine and chlorinated pyridines are toxic to the bacteria that convert ammonium ions to nitrate. Thus, these types of inhibitors delay conversion of ammonium nitrogen to nitrate by specifically inhibiting the activity of the soil bacteria.
A controlled release mechanism can also be achieved by combining soluble fertilizers with carriers such as glass, diatomaceous earth, waxes, parafins, polymers or resins. One of these products is based on formulating micronutrient ingredients with slowly soluble glass and fritted materials. These materials are made by mixing trace elements of iron, zinc, manganese, copper, boron, or molybdenum with silicates, borates, or phosphates used to make glass. The mixture is then homogenized in a smelting process and the resultant solid mass is shattered and ground to the finished product.
Another product is based on mixing fertilizer ingredients with a suitable binder to produce a highly compacted product sold as a spike or tablet.